An alternative to darning sock heels & toes. Knit leg, & top of foot only in whatever yarns you like. Then knit heel, sole & toe with yr sturdy sock yarn. Kitchener stitch toe & top of foot together. Preferably use a different coloured yarn to stitch side seams. When the need arises, unpick seam & reknit heel & sole. The top sock is on its 3rd incarnation!